Adhesives for bonding handler wafers to device wafers and enabling mid-wavelength infrared laser ablation release

ABSTRACT

Methods are provided to form adhesive materials that are used to temporarily bond handler wafers to device wafers, and which enable mid-wavelength infrared laser ablation release techniques to release handler wafers from device wafers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/861,034, filed on Aug. 1, 2013, the disclosure of which isincorporated herein by reference. This application is related to U.S.patent application Ser. No. 14/266,966, filed concurrently herewith,U.S. patent application Ser. No. 13/687,531, filed on Nov. 28, 2012, andU.S. patent application Ser. No. 13/746,359, filed on Jan. 22, 2013, thedisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The field generally relates to wafer handling techniques and, inparticular, to methods for forming adhesive materials that are used totemporarily bond handler wafers to device wafers, and which enablemid-wavelength infrared laser ablation release techniques to releasehandler wafers from device wafers.

BACKGROUND

In the field of semiconductor wafer processing, increasing demands forlarge-scale integration, high density silicon packages has resulted inmaking semiconductor dies very thin. For example, for some applications,silicon (Si) wafers are backside ground and polished down to a thicknessof 50 μm or thinner. Although single crystal Si has very high mechanicalstrength, Si wafers and/or chips can become fragile as they are thinned.Defects can also be introduced by processing steps such asthrough-silicon via (TSV) processing, polishing, and dicing, whichfurther reduces the mechanical strength of a thinned wafer or chip.Therefore, handling thinned Si wafers presents a significant challengeto most automation equipment.

In order to facilitate the processing of a device wafer, a mechanicalhandler wafer (or carrier wafer) is usually attached to the device waferto enhance the mechanical integrity of the device wafer duringprocessing. When processing of the device wafer is complete, the handlerwafer needs to be released from the device wafer. The most commonapproach to handling a device wafer is to laminate the handler waferwith the device wafer using specially developed adhesives. Depending onfactors such as the processing steps, the product requirements, and thetype of the adhesive, various techniques have been used or proposed todebond or separate a thinned device wafer from a mechanical handlerwafer, including thermal release, chemical dissolving, mechanicalrelease, and laser ablation techniques.

A typical laser-assisted debonding process uses a polymeric adhesive(which is capable of sufficient absorption of energy in the UV (ultraviolet) spectrum) to bond a device wafer to a UV transparent glasshandler wafer. A laser ablation process is performed to ablate thepolymeric adhesive and achieve debonding between the glass handler waferand the device wafer. The use of a glass handler in the UV laserablation process has several drawbacks including poor thermalconductivity, incompatibility with certain semiconductor processingequipment, as well as high cost. Although the use of Si wafer handlerscan potentially overcome these drawbacks, silicon is not transparent tothe UV spectrum and therefore is not compatible with previouslydeveloped UV laser release technology.

SUMMARY

In general, embodiments of the invention include methods for formingadhesive materials that are used to temporarily bond handler wafers todevice wafers, and which enable mid-wavelength infrared laser ablationrelease techniques to release handler wafers from device wafers.

In one embodiment of the invention, a method for forming an adhesiveincludes mixing a quantity of filler particles, solvent, and surfactantto obtain a first mixture in which the filler particles are uniformlydispersed, mixing a quantity of adhesive material with the first mixtureto generate a second mixture, and vacuum mixing the second mixture toachieve a target viscosity that is effective for deposition coating thesecond mixture onto a substrate.

In one embodiment, the filler particles are formed of a material thatabsorbs infrared energy having a wavelength in a range of about 1.12 μmto about 5 μm, wherein such filler particles may be metallic particlesor carbon particles, for example.

In another embodiment, the filler particles are formed of a materialthat reflects infrared energy having a wavelength in a range of about1.12 μm to about 5 μm, wherein such filler particles may be alumina,boron nitride, and silica particles, ceramic spheres, or a combinationthereof, for example.

In another embodiment of the invention, a stack structure includes adevice wafer, a handler wafer, and a bonding structure disposed betweenthe device wafer and the handler wafer, wherein the bonding structurebonds the device wafer and the handler wafer together. The bondingstructure includes a release layer formed of conductive material, and abonding adhesive layer. The release layer is configured to besubstantially or completely vaporized by infrared ablation when exposedto infrared laser energy through the handler wafer to cause the releaseof the device wafer from the handler wafer as a direct result of theinfrared ablation of the release layer. The bonding adhesive layerincludes filler particles that are configured to reflect the infraredlaser energy away from the device wafer toward the release layer,wherein a wavelength of the infrared laser energy is in a range of about1.12 μm to about 5 μm.

In yet another embodiment of the invention, a stack structure includes adevice wafer, a handler wafer, and a bonding structure disposed betweenthe device wafer and the handler wafer, wherein the bonding structurebonds the device wafer and the handler wafer together. The bondingstructure includes a release layer formed of a bonding adhesive. Thebonding adhesive includes filler particles that are formed of a materialthat absorbs infrared laser energy having a wavelength in a range ofabout 1.12 μm to about 5 μm. The release layer is configured to besubstantially or completely vaporized by infrared laser ablation whenexposed to infrared laser energy through the handler wafer to cause therelease of the device wafer from the handler wafer as a direct result ofthe infrared ablation of the release layer.

These and other embodiments of the invention will be described in thefollowing detailed description of embodiments, which is to be read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is flow diagram of a method for processing and handling asemiconductor wafer according to an embodiment of the invention.

FIG. 2 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to an embodiment of the invention.

FIG. 3 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention.

FIG. 4 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention.

FIG. 5 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention.

FIG. 6 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention.

FIG. 7 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention.

FIG. 8 illustrates a method of forming an adhesive material that is usedto temporarily bond a handler wafer to a device wafer, and which enablesinfrared laser ablation release techniques, according to an embodimentof the invention.

FIG. 9 schematically depicts an apparatus to perform a laser debondingprocess to release a device wafer and handler wafer using mid-wavelengthinfrared energy, according to an embodiment of the invention.

FIGS. 10A and 10B illustrate laser scan patterns that may be implementedin the apparatus of FIG. 9 to perform a laser debonding process,according to embodiments of the invention.

FIG. 11 illustrates a method for effectively overlapping pulsed laserbean spots during an IR laser scan process to effectively ablate arelease layer, according to an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention will now be discussed in further detailwith regard to structures and methods for temporarily bonding handlerwafers to device wafers using bonding structures that include one ormore releasable layers that absorb mid-wavelength infrared radiation(“Mid-IR radiation”) to achieve wafer debonding by infrared radiationablation. For example, FIG. 1 is flow diagram that illustrates a methodfor processing and handling a semiconductor wafer according to anembodiment of the invention. Referring to FIG. 1, the method includesperforming a wafer bonding process by bonding a handler wafer (orhandler substrate) to a device wafer using a bonding structure thatcomprises a release layer (step 10). In one embodiment of the invention,the handler wafer is a Si handler wafer (or substrate) which is bondedto a Si device wafer, as the use of a mechanical Si handler waferenables compatibility with standard CMOS silicon wafer processingtechnologies. In other embodiments of the invention, the handler wafercan be formed of glass or other suitable materials that are transparentor semi-transparent (e.g., at least 50% transparent) to certainwavelengths in the infrared (IR) spectrum that is used for IR laserablation.

Moreover, release layers according to embodiments of the inventioninclude thin metallic layers and/or adhesive layers formed with metallicparticles, which serve as releasable layers that can be substantially orcompletely ablated (vaporized) using low-power Mid-IR radiation todebond the device and handler wafers. In particular, in one embodiment,a bonding structure which temporarily bonds a handler wafer to a devicewafer is formed with one or more release layers (e.g., thin metal film,adhesive with metallic particles) that are configured to strongly absorbMid-IR energy emitted from a pulsed IR laser, and provide high ablationefficiency with low ablation energy thresholds to enable quick releaseof handler wafers from device wafers. Indeed, with these bondingstructures, an ultra-short pulse of Mid-IR energy from the IR laser canbe readily absorbed by the release layer(s) (constrained in a veryshallow depth within the bonding structure) to thereby quickly andefficiently vaporize at least a portion of the release layer at aninterface of the bonding structure and the handler wafer and therebyrelease the handler wafer from the device wafer. Various structures andmethods for bonding handler wafers to device wafers will be described infurther detail below with reference to FIGS. 2-8.

Referring again to FIG. 1, once the wafer bonding process is complete,standard wafer processing steps can be performed with the handler waferattached to the device wafer (step 11). For instance, in one embodimentof the invention, the handler wafer is bonded to a BEOL(back-end-of-line) structure formed on an active surface of the devicewafer. In this instance, standard wafer processing steps such asgrinding/polishing the backside (inactive) surface of the device waferto thin the device wafer can be performed. Other wafer processing stepsinclude forming through-silicon-vias through the backside of the devicewafer to the integrated circuits formed on the active side of the devicewafer. Additional process steps may be employed to deposit thin films(such as, but not limited to, SiO2 and/or Si3N4) that by means ofcompressive or tensile force on the silicon substrate and/or handlerwafer, help to minimize silicon active wafer and/or bonded pairnon-planarity (or warp).

In other embodiments, the device wafer (having dicing tape on a surfacethereof) may be subject to a wafer dicing process with the handler waferattached such that an individual die, or multiple dies, can be held bythe temporary handler wafer for die assembly or other processes wherethe dies are assembled to a substrate or another full thickness die, andthen released in subsequent operations such as post assembly or postunderfill. During these processing steps, the handler wafer will impartsome structural strength and stability to the device wafer, as isreadily understood by those of ordinary skill in the art.

A next step in the illustrative process of FIG. 1 involves performing alaser ablation wafer debonding process to release the device wafer fromthe handler wafer (step 12). In one embodiment, this process involvesirradiating the bonding structure through the handler wafer usingmid-wavelength IR energy to laser ablate a release layer of the bondingstructure and release the device wafer. More specifically, in oneembodiment, the process involves directing a pulsed Mid-IR laser beam atthe handler wafer, and scanning the pulsed Mid-IR laser beam accordingto a predetermined scan pattern to laser ablate a release layer of thebonding structure. Laser ablation of the release layer comprisesubstantially or completely vaporizing at least a portion of the releaselayer (e.g., a thin metallic layer and/or an adhesive layer withmetallic particles) at an interface between the release layer and thedevice wafer, to enable release of the device wafer from the handlerwafer. In one embodiment of the invention, a Mid-IR laser ablationprocess is implemented using an infrared laser beam that emits Mid-IRradiation with a wavelength in a range of about 1.12 μm to about 5 μm,and more preferably, in a range of about 1.12 μm to about 3 μm. Variousembodiments of a Mid-IR laser ablation process will be described infurther detail below with reference to FIGS. 9-11.

Referring again to FIG. 1, once the Mid-IR laser ablation process iscomplete and the device wafer is released from the handler wafer, a postdebonding cleaning process can be performed to remove any remainingadhesive material or other residue (resulting from the ablation of thebonding structure) from the device wafer (step 13). For example,cleaning process can be implemented using a chemical cleaning process ora wet cleaning process to remove any polymer based adhesive material.Other suitable cleaning methods to remove residue of the ablated bondingstructure can be used, which are known to those of ordinary skill in theart.

The use of Mid-IR radiation to perform a laser ablation processaccording to embodiments of the invention provides many advantages ascompared to using Far-IR radiation (greater than 5 microns) for laserablation. For example, a laser ablation process using Mid-IR radiationis compatible with both Si and glass handlers and other handlers thatare formed of materials that are transparent to Mid-IR radiation. Incontrast, glass handlers are not transparent in the Far-IR spectrum and,consequently, cannot be utilized with Far-IR laser ablation techniques.Moreover, a laser ablation process using Mid-IR radiation is compatiblewith stress compensation layers (e.g., silicon oxide or silicon nitridelayers) which are formed on thin handler wafers to prevent warping ofthe thin handler wafers during semiconductor processing stages.Furthermore, as compared to Far-IR radiation, the shorter wavelength ofMid-IR radiation enables higher absorption rates in thin release layersand thus, requires a much lower ablation threshold (e.g., 10 times lowerenergy) to achieve effective ablation (vaporization or removal) of therelease layers. Another advantage of using Mid-IR radiation forablation, as compared to Far-IR radiation, is that commerciallyavailable dicing tape products are transparent to Mid-IR radiation. Assuch, during a laser ablation process, when a layer of dicing tape isdisposed on a surface of a device wafer, the dicing tape will not sufferthermal damage during a Mid-IR laser ablation process.

FIG. 2 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to an embodiment of the invention. More specifically, FIG. 2is a schematic side view of a stack structure 20 comprising a devicewafer 21, a handler wafer 22, and a bonding structure 23. The bondingstructure 23 comprises an adhesive layer 24, and a release layer 25.FIG. 2 further illustrates a Mid-IR laser 14 that emits an IR laser beamat the handler wafer 22 to irradiate a portion of the release layer 25,resulting in a laser-ablated region 16.

In one embodiment of the invention, Mid-IR laser 14 emits a pulsedinfrared laser beam to laser ablate the release layer 25, wherein theMid-IR laser 14 emits a mid-wavelength infrared laser beam with awavelength in a range of about 1.12 μm to about 5 μm, and morepreferably, in a range of about 1.12 μm to about 3 μm. The handler wafer22 may be a silicon wafer or a glass wafer, wherein both silicon andglass are at least approximately 50% transparent to IR radiationwavelengths of 1.12 μm to about 3 μm. As such, the IR laser beam willpenetrate the handler wafer 22 and irradiate the release layer 25.

In one embodiment of the invention, the release layer 25 is formed of ametallic material having properties such as being reactive (not inert),soft, and having a relatively low melting point. For example, therelease layer 25 may be formed of metallic materials such as aluminum(Al), tin (Sn) or zinc (Zn). In other embodiments, the release layer 25is formed of carbon materials such as carbon nanotubes and graphene, forexample. Depending on the material used for the release layer 25, therelease layer 25 can be formed with a thickness in a range of about 5nanometers to about 400 nanometers. For example, in one embodiment inwhich the release layer 25 is formed of a metallic material such asaluminum, the release layer 25 can be formed with a thickness in a rangeof about 5 nanometers to about 200 nanometers. In an embodiment in whichthe release layer 25 is formed of carbon material, the release layer 25can be formed with a thickness of about 400 nanometers or less.

The ablation threshold of Mid-IR laser irradiation (level of exposureand time of exposure) for vaporizing the release layer 25 will varydepending on the thickness and type of material used to form the thinrelease layer 25. In all instances, the thin release layer 25 isconfigured to substantially absorb (and not reflect or transmit) theMid-IR laser energy, so that ablation of the thin release layer 25occurs.

In one embodiment, the adhesive layer 24 may be formed of any suitablepolymer adhesive material that may or may not be capable of sufficientlyabsorbing the Mid-IR energy output from the IR laser 14. Irrespective ofthe IR absorption ability of the adhesive layer 24, in one embodiment ofthe invention, the release layer 25 is configured (in materialcomposition and thickness) to intensely absorb the Mid-IR energy andserve as a primary releasable layer of the bonding structure 23, whichis ablated by the IR laser energy. The release layer 25 improves thelaser ablation efficiency and thus, reduces the ablation threshold ofthe bonding structure 23 (as compared to a bonding structure that usesan adhesive layer alone). In one embodiment of the invention, therelease layer 25 is irradiated with infrared energy sufficient to fullyvaporize (ablate) a portion of the release layer 25 that is exposed tothe Mid-IR energy, or at lease fully vaporize the material of therelease layer 25 at the interface between the handler wafer 22 and therelease layer 25 so as release the handler wafer 22.

Moreover, in an alternate embodiment of the invention, the bondingstructure 23 is irradiated with Mid-IR energy sufficient to fullyvaporize (ablate) at least a portion of the thin release layer 25 thatis exposed to the Mid-IR energy, as well as vaporize, denature,carbonize, or otherwise ablate and at least a portion of the adhesivelayer 24 at an interface between the adhesive layer 24 and the portionof the release layer 25 that is irradiated and ablated. In other words,in the bonding structure 23 shown in FIG. 2, the portion of the releaselayer 25 that is irradiated by the Mid-IR laser 14 is heated andvaporized, and this heating and ablation of the thin release layer 25results in heating of the surrounding material of the adhesive layer 24(at the interface between the irradiated release layer 25 and adhesivelayer 24), which causes ablation of the adhesive layer 24. In addition,depending on the IR absorption properties of the material used to formthe adhesive layer 24, ablation of the adhesive layer 24 is furtherachieved by any additional heating that is due to absorption of theMid-IR energy by the adhesive layer 24.

In one embodiment of the invention, the stack structure 20 can befabricated as follows. Initially, the release layer 25 is formed on asurface of the handler wafer 22. For example, the release layer 25 maybe formed by depositing a layer of metallic material (e.g., Al) using astandard technique such as chemical vapor deposition (CVD), physicalvapor deposition (PVD), or atomic layer deposition (ALD). In otherembodiments, the release layer 25 can be formed by growing or otherwiseplacing a layer of carbon material (e.g., carbon nanotubes, graphenelayer, etc.) on a surface of the handler wafer 22 using knowntechniques.

A next step includes forming the adhesive layer 24 on the release layer25. The adhesive layer 24 can be formed using known materials anddeposition techniques. For instance, the adhesive layer 24 can be formedof any suitable polymeric adhesive material, high-temperaturethermoplastic polyimides, BCB, acrylics, epoxies, or other bondingadhesive materials that are suitable for the given application. Theadhesive layer 24 can be formed by spin coating the adhesive material onthe release layer 25, and thermally baking the adhesive material to formthe adhesive layer 24. Thereafter, a standard bonding process isimplemented to bond the handler wafer 22 (with the bonding structure 23)to the device wafer 21.

FIG. 3 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention. More specifically,FIG. 3 is a schematic side view of a stack structure 30 which is similarto the stack structure 20 of FIG. 2, except that in the embodiment ofFIG. 3, a protective layer 32 is disposed between the bonding structure23 and the device wafer 21. The adhesive layer 24 and the release layer25 shown in FIG. 3 may be formed of the same or similar materials asdiscussed above with reference to FIG. 2.

In the embodiment of FIG. 3, the protective layer 32 serves to protectthe device wafer 21 from being irradiated with the infrared energyemitted from the Mid-IR laser 14 during a laser ablation process. Morespecifically, the protective layer 32 is configured (in materialcomposition and thickness) to reflect incident Mid-IR laser energy awayfrom the device layer 21 back toward the release layer 25. In oneembodiment of the invention, the protective layer 32 may be formed of aninert metallic material such as titanium, chromium, gold or copper, witha thickness that is sufficient to reflect the Mid-IR energy (thickerthan a skin depth of the protective layer 32 at the given Mid-IR laserwavelength). For example, the protective layer 32 may be formed of ametallic material such as Ti with a thickness in range of about 50 nm toabout 500 nm. In the embodiment of FIG. 3, the ablation efficiency ofthe irradiated portion of the release layer 25 (and adhesive material)in the laser-ablated region 16 is further enhanced by the additionalMid-IR irradiation reflected back from the protective layer 32, asschematically depicted in FIG. 3.

In one embodiment of the invention, the stack structure 30 can befabricated using similar methods as discussed above with reference toFIG. 2. Initially, the release layer 25 is formed on a surface of thehandler wafer 22 followed by forming the adhesive layer 24 on therelease layer 25, using materials and techniques as discuss above withreference to FIG. 2. Next, the protective layer 32 is formed on asurface of the device wafer 21 using suitable metallic materials andknown deposition techniques. Thereafter, a standard bonding process isimplemented to bond the handler wafer 22 (with the bonding structure 23)to the device wafer 21 (with the protective layer 32) to construct theresulting stack structure shown in FIG. 3.

FIG. 4 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention. More specifically,FIG. 4 is a schematic side view of a stack structure 40 which is similarto the stack structure 20 of FIG. 2, except that in the embodiment ofFIG. 4, a stress compensation layer 42 is formed on a surface of thehandler wafer 22 and disposed between the bonding structure 23 and thehandler wafer 22. The adhesive layer 24 and the release layer 25 shownin FIG. 4 may be formed of the same or similar materials as discussedabove with reference to FIG. 2. In the embodiment of FIG. 4, the stresscompensation layer 42 serves to prevent warping of stack structure 40,which can result due to lateral stresses that are applied to thedifferent layers of the stack structure 40 during the semiconductorfabrication processing stages that are performed to build the stackstructure 40 and/or process the device wafer 21 while attached to thehandler wafer 22.

In this embodiment, the stress compensation layer 42 is configured (inmaterial composition and thickness) to counteract stress forces thatcould otherwise be applied to the layers of the stack structure 40potentially causing the stack structure 40 to warp. In addition, thestress compensation layer 42 is configured (in material composition andthickness) to be transparent to the wavelength of the Mid-IR laserradiation used in the laser ablation process so that the Mid-IR energywill pass through the stress compensation layer 42 to irradiate therelease layer 25.

In one embodiment of the invention, the stress compensation layer 42 maybe formed of a silicon oxide material (e.g., SiO2) or a silicon nitridematerial (e.g., Si3N4), for example. The stress compensation layer 42 ispreferably formed with a thickness in a range of about 100 nm to about5000 nm, wherein the thickness will depend on the material used and theamount of stress counteraction force needed to prevent warping of thestack structure 40 for the given application. In general, while siliconsubstrates are relatively strong and are typically not subject towarping, relatively thin silicon substrates (e.g., a thinned silicondevice wafer) can be susceptible to warping. On the other hand, handlerwafers made of glass or other materials, are not as strong as siliconhandlers, and are more susceptible to warping. Thus, the implementationand composition of the compensation layer 42 will depend on factors suchas, for example, the material composition and thickness of the handlerwafer 22 and device wafer 21, and the nature of the semiconductorprocessing steps that are used to build the stack structure 40 andprocess the device wafer 21 for a given application.

In one embodiment of the invention, the stack structure 40 can befabricated using a similar method as discussed above with reference toFIG. 2. Initially, the stress compensation layer 42 is formed on asurface of the handler wafer 22, followed by sequentially forming therelease layer 25 and the adhesive layer 24 on the stress compensationlayer 42. These layers 42, 25 and 24 are formed using suitable materialsand known techniques, as discussed above. Thereafter, a standard bondingprocess is implemented to bond the handler wafer 22 (with the bondingstructure 23 and stress compensation layer 42) to the device wafer 21 toconstruct the resulting stack structure shown in FIG. 4.

FIG. 5 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention. More specifically,FIG. 5 is a schematic side view of a stack structure 50 which is similarto the stack structure 40 of FIG. 4, except that in the embodiment ofFIG. 5, the stress compensation layer 42 is formed on a surface of thehandler wafer 22 opposite the surface of the handler wafer 22 on whichthe bonding structure 23 is formed. The adhesive layer 24, release layer25, and stress compensation layer 42 shown in FIG. 5 may be formed ofthe same or similar materials as discussed above with reference to FIG.4. However, when fabricating the stack structure 50, the stresscompensation layer 42 is initially formed on a surface of the handlerwafer 22, followed by formation of the bonding structure 23 on anopposite surface of the handler wafer 22.

FIG. 6 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention. More specifically,FIG. 6 is a schematic side view of a stack structure 60 which is similarto the stack structure 20 of FIG. 2, except that in the embodiment ofFIG. 6, a reflective adhesive layer 62 is used in the bonding structure23. More specifically, in the embodiment of FIG. 6, the reflectiveadhesive layer 62 is formed of a polymer adhesive material, for example,that is premixed with IR reflecting particles. The IR reflectingparticles may comprise metallic particles (AL, Cu, etc.), or ceramicspheres, and/or other types of nanoparticles such as alumina, boronnitride, silica, etc., which serve to block or otherwise reflectincident Mid-IR radiation.

In this regard, the reflective adhesive layer 62 serves similarfunctions as the reflective layer 32 in the stack structure 30embodiment of FIG. 3. In particular, the reflective adhesive layer 62serves to protect the device wafer 21 from IR radiation. In addition,the reflective adhesive layer 62 serves to enhance the ablationefficiency of the release layer 25 in the laser-ablated region 16 due tothe additional Mid-IR irradiation that is reflected back to the releaselayer 25 from the reflective adhesive layer 62, thereby reducing theablation threshold of the release layer 25.

Moreover, when the reflective adhesive layer 62 is formed with metallicparticles, or other thermally conductive materials, the thermalconductivity of the adhesive layer 62 is increased. A thermallyconductive adhesive layer 62 advantageously serves to spread anddissipate heat in the stack structure 60 during various processingstages when fabricating the stack structure 60 and when processing thedevice wafer 21, and thereby enable high-power testing of the devicewafer 21 while bonded to the handler wafer 22. The reflective adhesivelayer 62 can be formed using methods as discussed below with referenceto FIG. 8, for example.

FIG. 7 schematically depicts a stack structure comprising a bondingstructure for temporarily bonding a device wafer to a handler wafer,according to another embodiment of the invention. More specifically,FIG. 7 is a schematic side view of a stack structure 70 comprising adevice wafer 21 and handler wafer 22, which are bonded together using anIR absorptive adhesive layer 72. In the embodiment of FIG. 7, theabsorptive adhesive layer 72 is formed of a polymer adhesive materialthat is premixed with metallic nanoparticles that improve the IRabsorption of the adhesive material. For example, the nanoparticles maybe formed of Sn, Zn, Al, carbon nanotubes or graphene, or a combinationthereof. The IR absorptive adhesive layer 72 may be formed by adeposition coating process, such as spin coating or spray coating, orsome other alternate form of deposition coating known in the art (or anycombination of deposition coating processes), wherein the polymeradhesive material with the premixed metallic nanoparticles is depositioncoated onto the surface of handler wafer 22 before bonding to the devicewafer 21.

In the embodiment of FIG. 7, the IR absorptive adhesive layer 72 servesas a releasable layer by infrared ablation of the adhesive layer 72, asshown in FIG. 7. Moreover, since the absorptive adhesive layer 72 isformed with thermally conductive materials, the thermal conductivity ofthe adhesive layer 72 is increased which is advantageous for reasonsdiscussed above. In other alternative embodiments, the adhesive layers62 and 72 shown in FIGS. 6 and 7 can be utilized in place of theadhesive layers depicted in FIGS. 2, 3, 4 and 5.

FIG. 8 illustrates a method of forming an adhesive material which can beused to temporarily bond a handler wafer to a device wafer, and whichsupports laser ablation release techniques, according to embodiments ofthe invention. An initial step in forming an adhesive material includesmixing filling particles (metallic and/or non-metallic particles) with asurfactant (or coupler), and a solvent until a uniformly dispersedmixture is obtained (block 80). Methods for mixing include tipsonication, bath sonication, three-roll mixing and other suitable mixingtechniques known in the art. The surfactant/coupler and solventmaterials ensure that the filler particles are well dispersed, and helpto obtain a stable suspension and uniform cured properties.

The type of filler particles used will vary depending on whether thetemporary bonding adhesive will be used as a reflective adhesive layer(e.g., layer 62, FIG. 6) or a releasable layer (e.g., absorptiveadhesive layer 72, FIG. 7). For example, when used as a reflectivelayer, the particles to be included in the adhesive material compriseparticles or nanoparticles such as alumina, boron nitride, silica,ceramic spheres, or other similar materials. When used as an IRabsorptive layer, the filler particles to be included in the adhesivematerial include, for example, carbon nanoparticles, aluminumnanoparticles, and/or other metallic or conductive nanoparticles.Moreover, the types of coupler/surfactant materials that are used willdepend on the types of filler particles, solvent materials, and bondingadhesive materials that are used. For example, the solvent used shouldbe capable of dissolving the bonding adhesive (polymer matrix) that isused.

A next step includes adding bonding adhesive material to the dispersedmixture of filler particles (block 82). The bonding adhesive materialcan be any commercially available bonding adhesive material (or matrixmaterial) that can be reformulated using techniques as described hereinto include filler particles that enable the formation of a temporarybonding adhesive that is laser-ablatable (removable), has enhancedthermal conductivity, or can serve as an IR reflecting layer. Suchbonding adhesive materials include high-temperature thermoplasticpolyimides, BCB, acrylics, epoxies, and other suitable adhesivematerials.

The bonding adhesive material and dispersed mixture of filler particlesare then vacuum mixed until a desired viscosity is obtained for spincoating (block 84). This process (blocks 80, 82, 84) results in auniform mixture of bonding adhesive material with filler particles. Forspin coating applications, a target viscosity of the temporary bondingadhesive is in a range of about 10³ Pa-s to about 10⁵ Pa-s, for example.Thereafter, the resulting temporary bonding adhesive material can bespin coated onto a release layer 25 (FIG. 6) or directly onto thesurface of a handler wafer 22 (FIG. 7), and then thermally cured untilall solvent material is baked out and the polymer material iscross-linked (block 86).

By way of example, an adhesive material according to one embodiment ofthe invention includes aluminum nanoparticles (e.g., 70 nm or less)added into a thermoplastic polyimide at volumetric loading in a range ofabout 1% to about 35%, or more preferably, about 5% to about 35%. Theloading range is dependent on the percolation threshold of theparticular material in the adhesive. More specifically, in one exampleembodiment, 10 grams of 70 nm aluminum particles, 1 mg of TritonX-100(commercially available solvent), and 10 g of PGMEA (commerciallyavailable surfactant) are sonicated until a uniform dispersion isobtained. Then 20 g of HD 3007 (commercially available adhesive (matrix)material) is added to the mixture and mixed in a high shear mixer with acowls blade. After a uniform mixture is obtained, the mixture is placedinto a vacuum mixer and mixed until the PGMEA is evaporated and adesired viscosity of 10000 Pa-s is obtained.

It is to be appreciated that the method of FIG. 8 may be used to createa thermally conductive bonding adhesive that can be used to temporarilybond a handler wafer to a device wafer, and also serve as a releaselayer by itself (FIG. 7) or in conjunction with another release layer.For example, the adhesive layers depicted in FIGS. 2, 3, 4, and 5 can bea thermally conductive bonding adhesive material with IR absorbingfiller particles (which is formed and deposited using methods asdiscussed above with reference to FIG. 8), and used as a release layerin conjunction with a thin conductive release layer made of metallic orcarbon materials.

Furthermore, the use of a thermally conductive bonding adhesive totemporarily bond a handler wafer to a device wafer also provides supportfor high power testing of chips (dies) of the device wafer while thedevice wafer is bonded to the handler wafer. More specifically, a stackstructure such as shown in FIG. 2, 3, 4, 5, 6 or 7, for example, havinga thermally conductive adhesive layer as part of the bonding structurecan serve as a thermally conducting layer that transfers heat from thedevice wafer to the handler wafer during high power testing of the chipson the device wafer. By way of specific example, an enhanced electricaltest for a given stack structure can be implemented, wherein a thinnedsemiconductor device wafer is tested using wafer level test probes,while heat is removed from the device wafer through the bondingstructure and silicon handler wafer to a cold plate or heat sink that isthermally coupled to the handler wafer. The stack structure providesmechanical support for built in self-test (BIST) procedures, and canalso provide full power and ground delivery using test probes toelectrically test the active chip circuits for frequency (speed) versusvoltage, as well as other electrical test evaluations of the chip. Atthe same time, the stack structure with a thermally conductive adhesivelayer provides enhanced cooling capabilities through the alternate sideof the thinned device wafer with heat removed/spread through a thermallyconductive adhesive layer and handler wafer to a cold plate or heat sinkthermally coupled to the handler wafer.

FIG. 9 schematically depicts an apparatus to perform a laser debondingprocess to release a device wafer and handler wafer using mid-wavelengthinfrared energy, according to an embodiment of the invention. Inparticular, FIG. 9 schematically illustrates an apparatus 90 for laserscanning a stack structure 100 comprising a handler wafer 102 and devicewafer 104, which are temporarily bonded using one of the exemplarybonding structures with a laser-ablatable (removable) release layer, asdiscussed herein. In general, the apparatus 90 comprises a Mid-IR lasersource 92, a beam shaper 94, a beam raster device 96 comprising mirrors96-1, 96-2, and a vacuum chuck 98. The components 92, 94 and 96 of theapparatus 90 are part of a laser scan system that is configured to scana pulsed IR laser beam over the surface of the handler wafer 102 using acertain scan pattern. The infrared laser scan system controls the laserablation scan process by controlling the power (energy density beam),the scan speed, and the pulse rate, for example, in a manner that issufficient to effectively ablate a release layer of a bonding structurewithin the stack 100. The parameters of the IR laser scan can varydepending on the bonding structure framework.

More specifically, the laser beam source 92 emits a pulsed IR laser beamhaving a wavelength in a range of about 1.12 μm to about 5 μm, and morepreferably, in a range of about 1.12 μm to about 3 μm. The beam shaper94 focusses the Mid-IR laser beam that is emitted from the laser source92. The focused laser beam is directed to the beam raster device 96,wherein the plurality of movable (rotating) mirrors 96-1 and 96-2 arecontrollably operated using known techniques to direct the pulsed Mid-IRlaser beam at the stack structure 100 and quickly scan (e.g., within 20seconds) the entire surface of the handler wafer 102 with the laser beamusing one of a plurality of suitable scan patterns.

For example, FIGS. 10A and 10B illustrate laser scan patterns that maybe implemented in the apparatus of FIG. 9 for performing a laserdebonding process, according to an embodiment of the invention. FIG. 10Aillustrates a spiral scan pattern, wherein the scan begins at the edgeof the handle wafer and may be employed with multiple scan passes at theperimeter of the handle wafer and then travels in spiral directiontowards the inner center of the handler wafer 102. This permits anyvapor product to be exhausted from the interface of the handler waferwithout causing a higher local pressure or damage to the circuit wafer.Further, FIG. 10B illustrates a serpentine scan pattern, wherein thelaser beam again can be scanned at the perimeter of the wafer and thenis scanned back and forth across the surface of the wafer handler 102starting from one side of the handler wafer 102 towards an opposite sideof the handler wafer 102. The laser beam scan patterns shown in FIGS.10A and 10B allow the entire surface of the handler wafer 102 to belaser scanned such that no previously scanned surface region of thehandler wafer 102 is scanned more than once for regions with underlyingactive circuits.

In other words, the scan patterns shown in FIGS. 10A and 10B ensure thata previously laser scanned region is not scanned again, as repeatedscanning of the same region can result in damage to the device waferwhen a laser beam is directed at regions in which the release layer(s)have already been vaporized. For the perimeter of the wafer (where noactive circuits are underlying), one or more passes of the laser beammay be required depending the angle of the laser and any edge effects orinefficiencies due to edge effect, reflection or losses in beam contactto the release layer. Alternatively, if more than one pass of laserdebonding is needed for release, a protective layer to avoid damage tothe active circuits can be employed as described in this application.

FIG. 11 illustrates a method for effectively overlapping pulsed laserbean spots during an IR laser scan process to effectively ablate arelease layer when performing a laser debonding process, according to anembodiment of the invention. As shown in FIG. 11, a more efficient laserscan ablation is obtained when there is some overlap of successive laserbeam spots L1 and L2 in a given scan direction as indicated by arrow S.As schematically illustrated in FIG. 11, with a pulsed laser beam scan,to obtain effective overlapping of successive laser beam spots L1 andL2, the scan speed of the laser should be less than or equal to one-halfthe spot diameter D times the pulse frequency, i.e.,

${{scan}\mspace{14mu}{speed}} \leq \frac{D \times {pulse}\mspace{14mu}{frequency}}{2}$Otherwise, if there is insufficient overlapping (or no overlapping) ofthe laser beam spots L1 and L2, there can be regions of the releasablelayer that are not properly irradiated and therefore, potentially notsufficiently ablated.

Referring back to FIG. 9, during a laser scan process, the stackstructure 100 is maintained in position on the vacuum chuck 98, wherebya vacuum system applies a vacuum suction force through the vacuum chuck98 to hold the stack structure 100 in place with the device wafer 102 incontact with the vacuum chuck 98. In one embodiment of the invention,the vacuum chuck 98 is configured to vibrate with ultrasonic ormegasonic energy so as to apply vibrational forces during and/or afterthe laser scan to assist in the release of the handler wafer 102 fromthe device wafer 104. After the IR laser scan is complete, the vacuumsystem places a second vacuum chuck (not specifically shown) in contactwith the handler wafer 102, and applies a vacuum suction force throughthe second vacuum chuck 120. The second vacuum chuck is lifted up with alifting device to pull the released handler wafer 102 away from thedevice wafer 104. The force required to pull the handler wafer 102 awayfrom the device wafer 104 is minimal because the release layer issubstantially or completely vaporized, which effectively results in therelease of the handler wafer 102 from the device wafer 104 by virtue ofthe laser scan ablation process.

Thereafter, the device wafer 104 can be transferred to a chemicalstation to etch or otherwise remove the residual temporary adhesivelayer or other bonding structure materials that remain on the surface ofthe device wafer 104 after the debonding process. Although not shown inFIG. 9, the apparatus 90 may further comprise an air handler,filtration/condensation system or exhaust system to remove and trapdebris and exhaust excess gases that are generated during the debondingprocess. It is to be understood that FIG. 9 is a generic, high-levelstructural depiction of a standard wafer-processing apparatus that canbe implemented or retrofitted to perform IR laser ablation and waferdebonding, as discussed herein.

Although embodiments have been described herein with reference to theaccompanying drawings for purposes of illustration, it is to beunderstood that the present invention is not limited to those preciseembodiments, and that various other changes and modifications may beaffected herein by one skilled in the art without departing from thescope of the invention.

We claim:
 1. A method for forming an adhesive to temporarily bond ahandler wafer to a device wafer, the method comprising: mixing aquantity of thermally conductive filler particles, solvent, andsurfactant to obtain a first mixture in which the thermally conductivefiller particles are uniformly dispersed; mixing a quantity of a polymeradhesive material with the first mixture to generate a second mixture;and vacuum mixing the second mixture to form a thermally conductivepolymer adhesive material having a target viscosity that is effectivefor deposition coating the thermally conductive polymer adhesivematerial onto a handler wafer or a device wafer, and wherein the vacuummixing removes the surfactant from the thermally conductive polymeradhesive material.
 2. The method of claim 1, wherein the fillerparticles are formed of a material that absorbs infrared energy having awavelength in a range of about 1.12 to about 5 μm.
 3. The method ofclaim 2, wherein the filler particles comprise metallic particles. 4.The method of claim 2, wherein the filler particles comprise carbonparticles.
 5. The method of claim 1, wherein the filler particles areformed of a material that reflects infrared energy having a wavelengthin a range of about 1.12 to about 5 μm.
 6. The method of claim 5,wherein the filler particles comprise one of alumina, boron nitride, andsilica particles, ceramic spheres, or a combination thereof.
 7. Themethod of claim 1, wherein mixing the quantity of thermally conductivefiller particles, solvent, and surfactant to obtain a first mixture isperformed using sonication mixing.
 8. The method of claim 1, wherein thetarget viscosity is in a range of about 10³ Pa-s to about 10⁵ Pa-s. 9.The method of claim 1, further comprising: coating a layer of thethermally conductive polymer adhesive material onto a surface of ahandler wafer by spin coating, spray coating or an alternate depositioncoating process; and thermally curing the coated layer to form athermally conductive bonding adhesive layer with the thermallyconductive filler particles to bond a device wafer to the handler wafer.10. The method of claim 9, wherein the thermally conductive bondingadhesive layer with the thermally conductive filler particles isconfigured to serve as a release layer which absorbs infrared energyhaving a wavelength in a range of about 1.12 to about 5 and which isvaporized by absorbing the infrared energy.
 11. The method of claim 1,further comprising: spin coating a layer of the thermally conductivepolymer adhesive material onto a surface of a handler wafer; andthermally curing the spin coated layer to form a thermally conductivebonding adhesive layer with the thermally conductive filler particles tobond a device wafer to the handler wafer.
 12. The method of claim 1,further comprising: spin coating a layer of the thermally conductivepolymer adhesive material onto a release layer formed on a surface of ahandler wafer; and thermally curing the spin coated layer to form athermally conductive bonding adhesive layer with the thermallyconductive filler particles to bond a device wafer to the handler wafer.13. The method of claim 12, wherein the thermally conductive bondingadhesive layer with the thermally conductive filler particles isconfigured to reflect infrared energy having a wavelength in a range ofabout 1.12 μm to about 5 μm.